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Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: 886–893, April 2010 Part B
The association between stratospheric weak polar vortex events
and cold air outbreaks in the Northern Hemisphere
Erik W. Kolstad,a*TarjeiBreiteig
a,c and Adam A. Scaifeb†
aBjerknes Centre for Climate Research, Bergen, Norway
bHadley Centre for Climate Prediction and Research, Met Office, Exeter, UK
cGeophysical Institute, University of Bergen, Bergen, Norway
*Correspondence to: Erik W. Kolstad, Bjerknes Centre for Climatic Research, Postboks 7810, 5020 Bergen, Norway.
E-mail: erik.kolstad@uni.no
†The Contribution of Adam A. Scaife was written in the course of his employment at the Met Office, UK and is published
with the permission of the Controller of HMSO and the Queen’s Printer for Scotland.
Previous studies have identified an association between temperature anomalies
in the Northern Hemisphere and the strength of stratospheric polar westerlies.
Large regions in northern Asia, Europe and North America have been found to
cool during the mature and late stages of weak vortex events in the stratosphere.
A substantial part of the temperature changes are associated with changes in the
Northern Annular Mode (NAM) and North Atlantic Oscillation (NAO) pressure
patterns in the troposphere. The apparent coupling between the stratosphere and the
troposphere may be of relevance for weather forecasting, but only if the temporal and
spatial nature of the coupling is known. Using 51 winters of re-analysis data, we show
that the development of the lower-tropospheric temperature relative to stratospheric
weak polar vortex events goes through a series of well-defined stages, including the
formation of geographically distinct cold air outbreaks. At the inception of weak
vortex events, a precursor signal in the form of a strong high-pressure anomaly over
northwest Eurasia is associated with long-lived and robust cold anomalies over Asia
and Europe. A few weeks later, near the mature stage of the weak vortex events,
a shorter-lived cold anomaly emerges off the east coast of North America. The
probability of cold air outbreaks increases by more than 50% in one or more of these
regions during all phases of the weak vortex events. This shows that the stratospheric
polar vortex contains information that can be used to enhance forecasts of cold air
outbreaks. As large changes in the frequency of extremes are involved, this process
is important for the medium-range and seasonal prediction of extreme cold winter
days. Three-hundred-year pre-industrial control simulations by 13 coupled climate
models corroborate our results. Copyright c
2010 Royal Meteorological Society
and Crown Copyright.
Key Words: stratosphere-troposphere interactions; climate models; natural variability; North Atlantic
Oscillation; Arctic Oscillation
Received 29 May 2009; Revised 11 February 2010; Accepted 2 March 2010; Published online in Wiley InterScience
4 May 2010
Citation: Kolstad EW, Breiteig T, Scaife AA. 2010. The association between stratospheric weak polar
vortex events and cold air outbreaks in the Northern Hemisphere. Q. J. R. Meteorol. Soc. 136: 886– 893.
DOI:10.1002/qj.620
Copyright c
2010 Royal Meteorological Society and Crown Copyright.
Weak Polar Vortex and Cold Air Outbreaks 887
1. Introduction
Cold air outbreaks (CAOs) are departures of cold air masses
into warmer regions. Over land, these events can lead to
deaths and damage (Mercer, 2003; Barnett et al., 2005;
Pinto et al., 2007). Over the ocean, CAOs are important for
a number of reasons: they give rise to mesoscale weather
phenomena such as polar lows (Bracegirdle and Gray, 2008),
they lead to enhanced heat and momentum fluxes from the
ocean to the air (Renfrew and Moore, 1999) and may
therefore influence the ocean circulation (Pickart et al.,
2003), and they cause rapid formation of sea ice in marginal
ice zones (Skogseth et al., 2004). In recent years it has
emerged that anomalies in the stratospheric circulation can
be associated with tropospheric CAOs (Thompson et al.,
2002; Cai and Ren, 2007; Scaife et al., 2008).
Normally, the extratropical stratosphere is characterised
by a strong westerly circumpolar flow. In winter, planetary
waves of tropospheric origin propagate continuously into
the stratosphere (Charney and Drazin, 1961), where they
break and exert a drag on the zonal flow (McIntyre and
Palmer, 1983; Polvani and Waugh, 2004). This violates
the geostrophic balance and induces a poleward drift of
air masses. At high latitudes, the air converges, sinks and
warms adiabatically. If there is severe wave-breaking, the
stratospheric zonal flow reverses, giving rise to stratospheric
sudden warmings (SSWs: Matsuno, 1971), which may last
for days to weeks (Limpasuvan and Hartmann, 1999).
After their first appearance in the upper stratosphere,
circulation anomalies are occasionally found at successively
lower levels (Matsuno, 1970; Lorenz and Hartmann, 2003).
After reaching the tropopause, the anomalies may impact
the troposphere through an interaction with synoptic-scale
eddies (Song and Robinson, 2004), or more directly through
induced meridional circulations. As a result, a negative
Northern Annular Mode (NAM: Thompson and Wallace,
2001) and North Atlantic Oscillation (NAO: Hurrell et al.,
2003) pattern may occur near the surface some weeks after
the first warming signal in the upper stratosphere (Baldwin
and Dunkerton, 2001; Baldwin et al., 2003; Limpasuvan
et al., 2004).
Negative NAM and NAO regimes in the troposphere
have a profound influence on the weather in large and
widespread regions of the Northern Hemisphere (NH)
(Kenyon and Hegerl, 2008). Atlantic and Pacific storm tracks
shift latitudinally (Hurrell and Van Loon, 1997; Baldwin and
Dunkerton, 2001), Greenland and Newfoundland warm
(Thompson et al., 2002), and the frequency and severity of
CAOs increase over large parts of east Asia (Chen et al.,
2005; Jeong and Ho, 2005), northern Eurasia (Scaife et al.,
2008) and eastern North America (Thompson and Wallace,
2001; Walsh et al., 2001; Cellitti et al., 2006). Over the ocean,
negative phases of the NAO, and positive height anomalies
over Greenland in particular, are associated with marine
CAOs over the Nordic Seas (Kolstad et al., 2009).
Motivated by the link that has been observed between
anomalous stratospheric events and the tropospheric
climate, we aim to provide a detailed description of
tropospheric cold anomalies in relation to such events.
Thompson et al. (2002) investigated the mean temperature
response during the first 60 days after the onset dates of
stratospheric anomalous vortex conditions. Here, we extend
their work by assessing the temperature development and
changes in the probability of CAOs at different stages
Table I. The official CMIP3 designations of the models that
were used in this study.
BCCR-BCM2.0
CCSM3
CGCM3.1(T63)
CNRM-CM3
ECHAM5/MPI-OM
GISS-ER
GFDL-CM2.1
IPSL-CM4
INM-CM3.0
MIROC3.2(medres)
MRI-CGCM2.3.2
PCM
UKMO-HadCM3
of stratospheric weak vortex events. We find that the
tropospheric temperature development goes through several
distinct and well-defined stages of stratospheric weak vortex
events and we identify CAOs over both continental and
oceanic regions. These results are corroborated by data from
300-year time slices of 13 coupled model simulations.
2. Data and methods
Daily mean fields from the National Centers for Envi-
ronmental Prediction/National Center for Atmospheric
Research (NCEP/NCAR) re-analysis (hereafter referred to
as NNR) data (Kalnay et al., 1996) were used throughout
the study. The analysis period was from the autumn/winter
of 1958 to the winter/spring of 2009.
Monthly mean data from 13 models in the World
Climate Research Programme’s (WCRP’s) Coupled Model
Intercomparison Project phase 3 (CMIP3) multi-model
dataset were also used. An analysis of the stratospheric
variability of these models is found in Cordero and Forster
(2006). The models used are listed in Table I (note that
we used a more recent BCCR-BCM model simulation, as
described by Otter˚
aet al. (2009)). These were the only models
that included 300-year time slices of pre-industrial control
simulations, with no anthropogenic or natural forcing. The
time slices were chosen arbitrarily from the years that were
available for download.
The most commonly used measure of stratospheric
variability is the NAM index. However, as both the spatial
structureandthetemporalvariabilityoftheNAMdiffered
greatly across the models, we defined a Vortex Strength Index
(VSI) as −ZP,where,ZP≡(Zcos ϕ)cos ϕ,Z=
Z−Z,Zis the geopotential height, Zis its climatological
mean, ϕis the latitude, and the sum was performed on all
grid points north of 65◦N. The reason for the minus sign is
that the vortex is weak when the pressure is high and vice
versa. Anomalies were formed by removing the date-wise
climatological mean for each grid point. To ensure that the
climatology was smooth, we applied a 31-day running-mean
filter. The highest pressure level (below 10 hPa, where some
of the models appeared to be unreliable) for which data was
available from all the models was 50 hPa, so this was used
as the stratospheric reference level for both the models and
the NNR. Baldwin and Thompson (2009) have shown that
Copyright c
2010 Royal Meteorological Society and Crown Copyright. Q. J. R. Meteorol. Soc. 136: 886– 893 (2010)
888 E. W. Kolstad et al.
Table II. SSW central dates since the ones identified by
Charlton and Polvani (2007), as derived from the NNR.
18 Jan 2003
7 Jan 2004
21 Jan 2006
24 Feb 2007
22 Feb 2008
24 Jan 2009
a VSI in this form is practically identical to the daily zonal-
mean NAM index. The monthly VSI was computed from
the model data in a similar way, although the climatological
monthly means were not smoothed.
The analysis of this paper focuses on the temporal
development of the signals of anomalous temperature and
geopotential height. Both parameters were evaluated at a
pressure level of 850 hPa and their date-wise climatological
means and anomalies were found in the same manner as
for the area-averaged 50 hPa geopotential height anomalies
described above, although for each grid point separately.
Our analysis is centred on composites of days and
months for which the stratospheric vortex is weak. We
define Weak vortex days (WVDs) in the NNR as the days
for which the daily VSI falls below its overall wintertime
(December–March) 10th percentile. An alternative to this
method would be to remove the seasonal cycle (by using
the date-wise climatological 10th percentile as a threshold
instead), but this would have forced the WVDs to be equally
distributed among the winter months. Cold days are defined
as days with an 850 hPa temperature below its date-wise
climatological 10th percentile. When identifying cold days
we did remove the seasonal cycle, as the purpose of defining
cold days is to assess whether a given day is colder than
‘normal’. Weak vortex months (WVMs) and Cold months
in the models are defined with respect to the overall 10th
percentiles of the monthly mean anomalies.
To assess the sensitivity of the results to the choice of
method, the analysis was also done by compositing with
respect to a set of SSW central dates, as defined and identified
by Charlton and Polvani (2007) (hereafter referred to as
CP07). Six SSWs have occurred since CP07, yielding a total
of 31 SSWs since 1958. The new central dates, as derived
from the NNR, are listed in Table II.
3. Results
3.1. Weak vortex events
In Figure 1(a), a matrix of VSI values for each day in the
analysis period is shown. The values were grouped with
respect to deciles. The blue days are the WVDs as defined
above. The SSW central dates are shown using crosses. As
mentioned earlier, due to the way they were computed, the
density of WVDs is higher in midwinter than in early and
late winter. Figure 1(a) shows that this complies well with the
seasonal distribution of the SSW central dates. An advantage
of CP07’s approach is that all their events are independent,
and the study of lead/lag processes is therefore free of the
effects of artificial smoothing. The composite zonal-mean
of the zonal wind at 10 hPa and 60◦N relative to the SSW
central dates is shown in Figure 1(b). The rapid weakening
and gradual recovery of the polar vortex is clear. However,
a disadvantage of CP07’s approach, or indeed any approach
that selects a specific reference date for each event, is that one
must be certain that the correct date has been chosen in each
case. Otherwise, the temporal signal may be distorted. The
relative scarcity of observed SSWs adds weight to this issue.
The VSI-based approach, in which each WVD is regarded as
a separate ‘event’, leads to runs of days and a smoothing of
the temporal signal. However, it is simple and sensitive to
only one apriorichoice: the selection of a threshold value for
WVDs. The composite 50 hPa polar cap geopotential height
anomalies relative to WVDs are shown in Figure 1(c). The
symmetrical evolution of the height anomalies about the
WVDs is a result of the smoothing introduced by the VSI-
based approach. The symmetry also shows that the WVD
approach is biased towards the middle date of longer events.
We also note that in composites of WVDs, persistent events
are given more weight than transient ones.
3.2. Tropospheric signature
In this section, we analyse temporal developments in the
troposphere throughout the life cycles of stratospheric weak
vortex events using both CP07’s approach and the VSI-
based approach, with an emphasis on cold anomalies. To
simplify the notation, we build loosely on the terminology
of Limpasuvan et al. (2004). They examined the evolution of
wave activity fluxes and atmospheric pressure fields in several
sub-periods of SSW life cycles. We define the following
phases of weak vortex events: Precursor (45–31 days before
the central dates and WVDs), Onset (30–16 days before
same), Growth (15–1 days before same), Peak (0 – 14 days
after same), Mature (15–29 days after same), Decline
(30–44 days after same) and Decay (45– 59 days after
same). Note that the developments that are seen in Fig. 9
of Limpasuvan et al. (2004) are not necessarily directly
comparable to the developments in our time intervals.
In Figure 2, the development of the 850 hPa geopotential
height and temperature anomalies relative to both the SSW
central dates (Figure 2(a)) and WVDs (Figure 2(b)) are
shown. In the early stages (Precursor, Onset and Growth),
positive height anomalies centred over northwest Eurasia
and negative anomalies near the Bering Strait are found.
This corresponds to a pattern that has been found to
favour stratospheric warmings through an enhancement of
upward-propagating tropospheric wave-number-one waves
(Kuroda and Kodera, 1999; Garfinkel et al., 2010). It
is therefore thought to be a tropospheric precursor of
warmings aloft. Cold anomalies arise over north-eastern Asia
through anomalous northerly wind components, resulting
in southward advection of cold, Arctic air masses. Cold
anomalies are also found over Europe, where there are
anomalous easterlies. These winds relate to the anomalous
ridge over northwest Eurasia. To our knowledge, these
cold anomalies, which appear too early to be affected by
downward propagation of the negative NAM-like signals
after SSWs, have not been documented previously in
relation to weak vortex events. Bueh and Nakamura (2007)
document similar temperature patterns in response to
the so-called Scandinavian Pattern, which resembles the
anomalous height pattern found in Figure 2(b) (Precursor
stage).
By the Growth and Peak stages in the WVD framework
(Figure 2(b)), an NAO-like anomaly has appeared. This
Copyright c
2010 Royal Meteorological Society and Crown Copyright. Q. J. R. Meteorol. Soc. 136: 886– 893 (2010)
Weak Polar Vortex and Cold Air Outbreaks 889
10
30
50
70
90
(a)
Year
(b)
Days relative to SSW central dates
Zonal wind (ms-1)
(c)
400
300
200
100
Z
P anomaly (m)
−30 −20 −10 0 10 20 30
Days relative to WVDs
−30 −20 −10 0102030
0
10
20
30
1 Dec 1 Jan 1 Feb 1 Mar
1960
1970
1980
1990
2000
2009
Percentile
Figure 1. (a) The daily VSI, sorted and grouped with respect to deciles. The SSW central dates based on the algorithm of Charlton and Polvani (2007) are
marked with crosses. (b) The composite daily mean zonally averaged zonal wind at 60◦N and 10 hPa on each day relative to SSW central dates. (c) The
composite daily polar-cap 50 hPa geopotential height (ZP) anomalies on each day relative to WVDs.
−2.5 −1.5 −0.5 0.5 1.5 2.5
(a)
(b)
PeakGrowthOnsetPrecursor Mature Decline Decay
Figure 2. Composites of 850 hPa geopotential height anomalies (in m with solid contours, positive in black, negative in grey, contour interval 10 m,
zero contour omitted) and 850 hPa temperature anomalies (in K with filled contours, with white contours along the values specified on the colour bar)
relative to (a) SSW central dates and (b) WVDs, averaged over the specified time intervals. The region shown is the Northern Hemisphere north of 30◦N,
with Eurasia to the right and North America to the left.
leads to an anomalous northerly flow and cold anomalies
in northern Europe. This is consistent with an increased
frequency of marine CAOs in the Nordic Seas region
under negative NAO conditions (Kolstad et al., 2009).
As the pressure anomalies are contained primarily in the
Atlantic sector by this time, the cold anomalies in Asia
diminish in magnitude. At the same time, cold anomalies
appear on the east coast of North America. Corresponding
warm anomalies over Canada and the Mediterranean/North
Africa complete the quadrupole pattern of temperature
anomalies that are associated with the NAO (Stephenson
and Pavan, 2003). In the Mature, Decline and Decay stages,
the NAO pattern gradually weakens, and the most prominent
cold anomalies are found in Asia and Europe. This is
consistent with findings from previous studies (Baldwin
and Dunkerton, 2001; Thompson et al., 2002; Chen et al.,
2005).
Copyright c
2010 Royal Meteorological Society and Crown Copyright. Q. J. R. Meteorol. Soc. 136: 886– 893 (2010)
890 E. W. Kolstad et al.
0 0.25 0.5 0.75 1.25 1.5 1.75 2 2.25 2.5
PeakGrowthOnsetPrecursor Mature Decline Decay
(a)
(b)
Figure 3. The composite number of cold days divided by the date-wise climatological mean number of cold days for each grid point relative to (a) SSW
central dates and (b) WVDs, during the specified time intervals. Otherwise the plotting conventions are as in Figure 2.
(a)
(b)
0 0.25 0.5 0.75 1.25 1.5 1.75 2 2.25 2.5−2.5 −1.5 −0.5 0.5 1.5 2.5
1959–1983
1984–2009
(d)
(c)
Mature +
Decline
Growth +
Peak
Precursor +
Onset
Mature +
Decline
Growth +
Peak
Precursor +
Onset
Figure 4. (a), (b) Plotting conventions as in Figure 2. (c), (d) Plotting conventions as in Figure 3. The composite averages were computed relative to
WVDs for the two halves of the NNR period, with the period 1959–1983 in (a), (c) and 1984– 2009 in (b), (d).
To summarise, Figure 2 shows temporally and geograph-
ically distinct cold anomalies throughout the life cycle of
weak vortex events. The cold anomalies were found using
the CP07 method (Figure 2(a)) and using the WVD method
(Figure 2(b)), although the exact timing and relative ampli-
tudesdifferslightly.
3.3. Relative frequency of stratosphere-related cold air
outbreaks
Figure 2 was based on changes to the mean temperature
field, with no regard to its extreme values. We now define
the quantity αas the fractional change in the number
of cold days (days for which the 850 hPa temperature
anomaly falls below its date-wise 10th percentile) with
respect to climatology. If α=1.5, cold days are 50%
more likely than normal. As this parameter is only
concerned with cold extremes, the evolution of α,which
is shown in Figure 3, provides a useful complement to
Figure 2.
In the early stages of weak vortex events, i.e. during the
Precursor and Onset stages, α>1.5 most consistently in
Asia and Europe. By the Growth stage, α>1.5offthecoast
of North America in the WVD framework. In the later
Mature and Decline stages, the strongest cold anomalies are
again confined to Asia and Europe. It is only in Asia that
α>1.75 during all the periods shown. The large fractional
changes in Figure 3 show that the frequency of cold days
is affected strongly around the time of sudden stratospheric
warmings.
3.4. Robustness of the results
In Figure 4, the same analysis that was used to produce
Figures 2(b) and 3(b) was applied to the two halves of the
analysis period. Note that we averaged over longer time
periods than in Figures 2 and 3. In general, the features
shown previously for both the mean and extreme events are
robust to this halving of the data period. High pressure is
observed over northwest Eurasia prior to the weak vortex
events, and a subsequent negative NAO-like pattern is seen.
Similarly, cold anomalies are found over Asia, Europe and
near the east coast of North America. The perhaps largest
difference between the two periods is found over the Pacific
in the Mature/Decline phases. In the first part of the period,
an anomalous low over the Aleutian Islands (Figure 4(a)) is
associated with advection of cold, continental air out over
the Pacific (Figure 4(c)). The large Pacific high anomaly in
the latter part of the period (Figure 4(b)) is associated with
Copyright c
2010 Royal Meteorological Society and Crown Copyright. Q. J. R. Meteorol. Soc. 136: 886– 893 (2010)
Weak Polar Vortex and Cold Air Outbreaks 891
a 75% increase in the number of cold days along the west
coast of North America (Figure 4(d)).
As an additional test of robustness, we examined monthly
mean data from 13 coupled climate models. This part of
the analysis is done using the WVM framework. Some
of the CMIP3 models have low model tops and many of
them underestimate the stratospheric variance (Cordero
and Forster, 2006). The models will therefore be used to
evaluate the temporal and spatial variability rather than the
exact amplitudes of the anomalies. Based on the symmetry
of Figure 1(c), it is natural to define that WVMs correspond
to the Growth and Peak phases of weak vortex events.
Figure 5(a) shows the 850 hPa geopotential height and
temperature anomalies during WVMs (Growth/Peak phase),
as well as during the preceding (Precursor/Onset phase) and
the succeeding (Mature/Decline phase) months. The initial
cold anomaly in Asia, the westward shift of the northwest
Eurasia warm anomaly and the appearance of cold anomalies
in Europe and off the east coast of North America are all
seen in the model ensemble around weak vortex months.
The temperature developments are consistent with the
anomalous northerly and easterly flow that is associated
with the pressure anomalies, whereas the overall westward
progression of the temperature pattern indicates further
potential for seasonal predictability.
In Figure 5(b), the changes to the probability of cold
months in the different stages are shown. The following cold
anomalies are associated with a higher than 50% increase
in the number of cold months: (1) The cold anomaly over
Asia in the Precursor/Onset and Growth/Peak phases, (2) the
cold anomaly over northern Europe in the Growth/Peak
and Mature/Decline phases, and (3) the cold anomaly over
north-eastern North America in the Growth/Peak phases.
Qualitatively, the features in Figure 5 are in good agreement
with Figures 2 and 3, although the magnitudes of the
anomalies are generally weaker. This is at least partly due to
the much larger sample size of the model data.
4. Concluding remarks
The relationship between stratospheric weak vortex events
and tropospheric developments, and cold air outbreaks
(CAOs) in particular, were investigated using 51 winters of
re-analysis data and a set of coupled climate models. We
found large increases in the frequency of cold air outbreaks
(Figure 3) that coincide geographically with the regions of
mean temperature change (Figure 2). The probability of
CAOs was found to increase: (1) by 75% or more in some
regions of northern Asia throughout the life cycle of weak
vortex events (from the Precursor phase to the Decay phase),
(2) by 50% or more in some regions of Europe (from the
Onset phase to the Decline phase), and (3) by 50% or more
in the Peak phase off the east coast of North America.
Changes in the frequency of cold air outbreaks associated
with the stratosphere are therefore large compared to the
climatological incidence of CAOs. Such substantial changes
make this signal important for the long-range forecasting
of the likelihood of CAOs. If the signal is predictable, then
there will be an associated predictability of CAOs. However,
if it is unpredictable, then it represents an important limit
on the long-range predictability of CAOs.
A potential obstacle to the predictability of CAOs based
on the state of the stratospheric vortex is the fact that
many of the cold anomalies seen in Figure 3 occurred
(a)
(b)
−2.5 −1.5 −0.5 0.5 1.5 2.5
0 0.25 0.5 0.75 1.25 1.5 1.75 2 2.25 2.5
Mature +
Decline
Growth +
Peak
Precursor +
Onset
Figure 5. (a) The 13-member climate model ensemble average 850 hPa
temperature and geopotential height anomalies, and (b) relative probability
of cold months, one month before, during and one month after WVMs.
(a) Plotting conventions as in Figure 2. (b) Plotting conventions as in
Figure 3.
before the SSW central dates and WVDs. The early CAOs in
Europe and Asia were associated with the perhaps clearest
precursor of stratospheric weak vortex events, a high-
pressure anomaly centred over the northwestern edge of
Eurasia in the Precursor, Onset and Growth phases. Although
its location changed with time, this positive height anomaly
persisted for all the phases and was confined to high latitudes
in the Atlantic sector. More work is therefore needed to
address the chain of cause and effect and to investigate
tropospheric precursors of weak vortex events, adding to
existing studies of troposphere – stratosphere interactions
(Kuroda and Kodera, 1999; Chen et al., 2003; Polvani and
Waugh, 2004; Reichler et al., 2005; Scaife et al., 2005; Cohen
et al., 2007; Martius et al., 2009; Mukougawa et al., 2009;
Garfinkel et al., 2010).
We did not directly address the issue of cause and
effect of CAOs in this paper, but interestingly, we
found a hemisphere-wide pattern of lower-tropospheric
temperature signals both before and after weak vortex
events. In general, such temperature signals are associated
with pressure anomaly dipoles in the form of anomalous
ridges upstream (such as the precursory high anomaly over
northwest Eurasia) and anomalous troughs downstream of
the cold anomalies. Such patterns lead to changes to the
flow, and the resulting temperature advections may well
act as positive feedback mechanisms, as documented for the
negative phase of the surface NAM (Thompson and Wallace,
2000). The association between pressure anomaly dipoles
and CAOs is known from previous studies (Konrad, 1996;
Walsh et al., 2001; Chen et al., 2005; Takaya and Nakamura,
2005; Cellitti et al., 2006; Kolstad et al., 2009). It is quite
possible that some of the regional CAOs identified in this
paper are at least partly set up or sustained by cold air
advection, as part of the chain of events outlined by Konrad
(1996).
Given the strong stratospheric link to many CAOs, it
couldbethatattentionneedstobepaidtothesimulation
of the stratosphere in climate models. However, parts of
Copyright c
2010 Royal Meteorological Society and Crown Copyright. Q. J. R. Meteorol. Soc. 136: 886– 893 (2010)
892 E. W. Kolstad et al.
our analysis were repeated with an ensemble of 13 coupled
climate models. Somewhat surprisingly, considering that
many of these models have low model tops and poorly
resolved stratospheres (Cordero and Forster, 2006), the
model results corroborated the relationships between the
weak vortex events and the cold anomalies listed above. This
may indicate that the main aspects of the tropospheric
temperature developments during the life cycle of the
stratospheric weak vortex events are associated with internal
processes in the troposphere and lower stratosphere, as
suggested by Polvani and Waugh (2004).
Acknowledgements
We wish to thank the editors and two anonymous reviewers
forcontributingtoanimprovedpaper.Wealsoacknowledge
the modelling groups, the Program for Climate Model
Diagnosis and Intercomparison (PCMDI) and the WCRP’s
Working Group on Coupled Modelling (WGCM) for
making available the WCRP CMIP3 multi-model dataset,
as well as NOAA/OAR/ESRL PSD for providing the
NCEP/NCAR re-analysis data. Erik Kolstad’s work was
funded by the Norwegian Research Council through its
International Polar Year programme and the project IPY-
THORPEX (grant number 175992/S30). Tarjei Breiteig’s
work was supported by the COMPAS project (grant number
165424), also funded by the Norwegian Research Council.
Adam Scaife was supported by the Joint DECC and Defra
Integrated Climate Programme – DECC/Defra (GA01101).
This is publication no. A259 from the Bjerknes Centre for
Climate Research.
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